1. Field of the Invention
The present invention relates to an MRI system and, more particularly, to a magnetostatic field generating magnet having an active magnetic shield.
2. Description of the Related Art
There are a permanent magnet, a normal conductive magnet, and a superconductive magnet as a magnetostatic field generating magnet provided with an MRI system. Since this type of magnetostatic field generating magnet is installed inside a hospital, a magnetic field leaking from the magnet has to be reduced to a minimum in order to eliminate a magnetically adverse effect to an ambience. The magnet is thus usually coated with a magnetic shield. Three magnetic shields, i.e., an iron yoke magnetic shield, an active magnetic shield and a hybrid magnetic shield have been put to practical use. Of these, the active magnetic shield necessitates no special shielding members and lightens the magnetostatic field generating magnet; therefore, it has an advantage in being placed inside a hospital and in particular it is effective in serving as a magnetic shield of a superconductive electrostatic field generating magnet.
A prior art magnetostatic field generating magnet and its magnetic shield, and an MRI system with such a magnet, will now be described with reference to the accompanying drawings. The magnetostatic field generating magnet is of a superconductive type.
FIG. 1 is a perspective, cutaway view of an active magnetic shield type magnet as disclosed in Jpn. Pat. Appln. KOKAI Publications Nos. 60-98344 and 60-123756. In FIG. 1, a magnetostatic shield generating magnet 60 includes a first superconductive coil assembly 61 for generating first magnetism as the main magnetism and a second superconductive coil assembly (active magnetic shield) 62 for generating second magnetism which is formed on the outer periphery of the assembly 61 and electrically connected in series thereto. These first and second assemblies are contained in a liquid helium tank 63 filled with liquid helium and held to an extremely low temperature of 4.2 K. To use the magnet 60 for an MRI system, the uniformity of magnetism is essential. In FIG. 1, reference numeral 64 indicates the axis of magnetism, 65 denotes the central plane of the MRI system, and 66 represent a normal temperature bore. The bore 66 is a space in which a subject to be examined is inserted.
FIG. 2 is a diagram showing an example of the relationship in coil arrangement between the first and second superconductive coil assemblies 61 and 62. Each of the assemblies has six superconductive coils. More specifically, the first assembly 61 includes three pairs of coils A and A', B and B' and C and C', while the second assembly 62 also includes three pairs of coils D and D', E and E' and F and F'. These coils are arranged along the axis 64, and the respective paired coils are symmetrical with regard to the central plane 64 which is perpendicular to the axis 64.
The above-described prior art superconductive magnetostatic field generating magnet having an active magnetic shield is operated as follows. The first and second superconductive coil assemblies 61 and 62 generate magnetism whose high-order magnetic components are substantially the same in intensity, and supply the uniformly synthesized magnetism to the central interaction space of the normal temperature bore 66. Since the direction of current flowing through the second assembly 62 is opposite to that of current flowing through the first assembly 61, the magnetism generated from the second assembly 62 cancels that from the first assembly 61 outside the magnet 60, thereby reducing in leaking magnetism.
The magnetism generated from the first superconductive coil assembly 61 is expressed by high-order magnetic components as follows: EQU B1=b01+b11+b21.vertline.b31.vertline. (1)
where B1 is intensity of the magnetism, b01 is that of zero-order magnetism, b11 is that of primary magnetism, b21 is that of secondary magnetism, and b31 is that of tertiary.
Similarly, the magnetism generated from the second superconductive coil assembly 62 is expressed by high-order magnetic components as follows: EQU B2=b02+b12+b22+b32 (2)
where B2 is intensity of the magnetism, b02 is that of zero-order magnetism, b12 is that of primary magnetism, b22 is that of secondary magnetism, and b32 is that of tertiary.
Since the corresponding high-order magnetism intensities b11 and b12, b21 and b22, b31 and b32, . . . are almost equal to each other in the above equations (1) and (2), the high-order terms become almost zero, with the result that the magnetism is highly uniformed in the normal temperature bore 66. Since, furthermore, the polarities of B1 and B2 are opposite to each other, the magnetism is canceled outside the magnet and the leaking magnetism is decreased.
The prior art superconductive magnetostatic field generating magnet 60 with the foregoing structure, has the following drawbacks. In the active magnetic shield of the magnet, the second superconductive coil assembly 62 through which the current flows in the direction opposite to that of the current flowing through the first superconductive coil assembly 61, is disposed on the outer periphery of the assembly 61. However, in order to greatly uniformize the magnetism in the bore 66, the corresponding high-order magnetic components of the magnetism generated by the first and second assemblies 61 and 62 need to be almost equal to each other, so that the magnet 60 is greatly lengthened.
According to the above publication No. 60-123756, in a 1.5 T magnet having a conventional active magnetic shield, the length of the magnet is 2.3 m and so is the diameter thereof. When the magnet is installed in a hospital, it is very difficult to carry the magnet through a passageway, especially a corner thereof. It is thus necessary to expand the passageway or make a room exclusively for the magnet.
The prior art superconductive magnetostatic field generating magnet is therefore disadvantageous in practicability because of great measurements. If the magnet 60 is long and narrow, a patient lying in the normal temperature bore 66 will strongly feel himself or herself enclosed or confined and reject the magnet.
Moreover, since the high-order magnetic components are cancel each other only by the second superconductive coil assembly 62 formed on the outer periphery of the first superconductive coil assembly 61, it is essentially difficult to make the high-order terms zero, and a uniform magnetic space cannot be enlarged.